The electrical properties of cardiac muscle determine the sequence of contraction of the intact heart and so are of considerable practical as well as fundamental significance. Analysis of the electrical properties of cardiac muscle is a necessary step in understanding the action potential, the propagation of the action potential, and the linkage between the action potential and contraction. The electrical properties arise in several specialized structures within cardiac muscle: the specific goals of this work are to determine the contribution of each specialized structure to the electrical properties of the tissue. Several preparations will be investigated since each preparation highlights certain aspects of cardiac function. The electrical role of each specialized structure will be determined by a combined anatomical, theoretical, and electrophysiological approach. Using separately determined structural parameters, we construct a general electrical theory with as few assumptions as possible. This theory is then solved, using perturbation techniques, in as physically meaningful manner as possible. The results of experiments in which current is applied to the tissue and potential recorded are interpreted with the separately determined theory and morphological parameters. The results of the analysis are the properties of the individual structures of the preparation. The analysis will be extended to cases where the tissue is active. In this way we will analyze the properties of individual structures associated with the spread of electrical activity and the initiation of contraction.